endpoint/modbusServer: v0.37.0+Modbus TCP slave endpoint. Accepts master connections; writes to Coil/Holding Register trigger rule chain messages.

Requires: rulego-components-iot (opens new window)

Configuration

Field	Type	Description	Default
server	string	Listen URL (tcp://host:port)	tcp://:502
unitId	int	Slave unit ID (0 = accept all)	0
maxClients	uint	Max concurrent connections	10
coils	uint	Coil area size (bits)	2000
registers	uint	Register area size (words)	2000

How It Works

1. In-memory register table: 4 areas (Coils/DI/HR/IR); master reads return table values
2. Write triggers rule chain: master writes to HR/Coil → message flows into rule chain
3. External write-back: rule chain updates registers via SetRegisters/SetCoils for master to read

Write Trigger Message

{"type": "holding_register", "unitId": 1, "addr": 100, "quantity": 2, "values": [1234, 5678], "clientAddr": "192.168.1.10:54321"}

Metadata injection: modbusType (coil/holding_register), clientAddr.

Use Cases

• SCADA bridge: master writes → trigger automation
• Virtual device: rule chain updates registers → master polls
• Write-triggered flows: PLC write → alert/logging/linkage